This invention relates to an improved heat pump and in particular, to an integrated heat pump and hot water system having a defrost cycle wherein the indoor coil is thermodynamically isolated from the system and energy from the hot water side of the system is used to evaporate refrigerant during the defrost cycle.
Integrated heat pump and hot water systems have been known and used in the art for some time. Typically, a desuperheater is placed in the discharge line of the refrigerant compressor and the exchanger configured so that superheat in the refrigerant leaving the compresssor is rejected into water passing through the exchanger. The amount of energy that can be provided to the water side of the system is usually limited to the amount of superheat available in the refrigerant leaving the compressor. This type of system furthermore cannot produce hot water unless the heat pump is delivering heating or cooling to a comfort zone. U.S. Pat. No. 4,311,498 to Miller shows a typical integrated heat pump and hot water system having a desuperheater for providing energy to the water side of the system.
In U.S. Pat. No. 4,598,557 to Robinson et al. there is disclosed a heat pump that is integrated with a domestic hot water system through means of a refrigerant to water heat exchanger that is operatively connected into the discharge line of the refrigerant compressor. Three different heat pump configurations can be obtained by selectively opening and closing a relatively large number of valves. In two configurations the heat pump delivers heating and cooling to an indoor comfort zone with or without heating water. In a third configuration the system is arranged to provide water heating only without any air conditioning. This is accomplished by manipulating the control valves to physically remove the indoor coil from the refrigeration side of the system. The refrigeration to water heat exchanger, in this third configuration, takes over the entire condensing load of the system and uses the heat of condensation to heat domestic water.
Although the Robinson et al. device represents an advancement in the art in that it provides for water heating during periods when air conditioning is not required, it never-the-less requires a good deal of additional equipment to produce three separate system configurations. Each configuration, because it is separated from the others, utilizes its own dedicated expansion device. More importantly, however, to establish any one configuration it is necessary to valve off entire sections of the refrigeration system. As a consequence, unused refrigerant in varying amounts becomes trapped in the isolated sections thereby making refrigeration management extremely difficult. While the proper amount of refrigerant might be available to operate the heat pump efficiently in one of the three configurations, the situation can change dramatically when the heat pump is changed over to one of the other configurations.
It should be further noted that the Robinson et al. compressor is unfortunately arranged to pump against the valves used to shut off various sections of the refrigeration system. High refrigerant pressures, coupled with normal wear on the valve parts, allows refrigerant to leak past the valve, further compounding refrigeration inventory problems. The Robinson et al. system, like other heat pump systems found in the prior art, must also employ inefficient strip heaters or the like to prevent cold air from being blown into a comfort air region during a defrost cycle.